Why Does Appending to a Nil Go Slice Double Its Capacity?

Unveiling the Mystery: Why Appending to a Nil Slice Expands Its Capacity by Two

When working with slices in Go, one may encounter an unexpected behavior: appending a single item to a nil slice not only extends its length but also doubles its capacity. To understand this phenomenon, let's delve into the inner workings of Go's slice implementation and the role of capacity.

Capacity: The Silent Partner

Capacity, a slice's hidden attribute, represents the maximum number of elements it can hold without reallocation. While slices can expand dynamically, this expansion involves allocating a new backing array and copying existing elements. By pre-allocating more space than immediately needed, Go optimizes performance by reducing the frequency of reallocations.

Nil Slice Anomaly

Appending an element to a nil slice initializes the slice with a backing array and sets both length and capacity to 1. To accommodate the new element, Go increases the capacity to 2, surpassing the actual demand. This extra capacity serves as a buffer to avoid future reallocations when more elements are appended.

Unexpected Zeros in Sliced Slices

Another puzzling observation is the appearance of zeros when re-slicing the extended slice using s2[0:2]. This behavior stems from the fact that while slices provide indexing beyond their length, accessing these unbound indices does not provoke a panic. However, these extra elements are not part of the slice's underlying array and should not be relied upon.

Focus on Length, Not Capacity

In practice, developers should primarily focus on slice length, as it indicates the number of valid elements. Capacity is primarily relevant for performance optimizations, and its specific value may vary depending on the Go environment.

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